Intervertebral Implant for transforaminal posterior lumbar interbody fusion procedure

ABSTRACT

An intervertebral implant for fusing vertebrae is disclosed. The implant has a body with curved, substantially parallel posterior and anterior faces separated by two narrow implant ends, superior and inferior faces having a plurality of undulating surfaces for contacting upper and lower vertebral endplates, and at least one depression at a first end for engagement by an insertion tool. The arcuate implant configuration facilitates insertion of the implant from a transforaminal approach into a symmetric position about the midline of the spine so that a single implant provides balanced support to the spinal column. The implant may be formed of a plurality of interconnecting bodies assembled to form a single unit. An implantation kit and method are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. patentapplication Ser. No. 09/848,178, filed May 3, 2001, the contents ofwhich is expressly incorporated herein by reference thereto.

FIELD OF THE INVENTION

[0002] The present invention is directed to an intervertebral implant,its accompanying instrumentation and their method of use. Moreparticularly, the present invention is directed to an intervertebralimplant and instrumentation for use in a transforaminal posterior lumbarinterbody fusion procedure.

BACKGROUND OF THE INVENTION

[0003] A number of medical conditions such as compression of spinal cordnerve roots, degenerative disc disease, herniated nucleus pulposis,spinal stenosis and spondylolisthesis can cause severe low back pain.Intervertebral fusion is a surgical method of alleviating low back pain.In posterior lumbar interbody fusion (“PLIF”), two adjacent vertebralbodies are fused together by removing the affected disc and insertingone or more implants that would allow for bone to grow between the twovertebral bodies to bridge the gap left by the disc removal.

[0004] One variation of the traditional PLIF technique is thetransforaminal posterior lumbar interbody fusion (T-PLIF) technique.Pursuant to this procedure, an implant is inserted into the affecteddisc space via a unilateral (or sometimes bilateral), posteriorapproach, offset from the midline of the spine, by removing the facetjoint of the vertebrae. The T-PLIF approach avoids damage to nervestructures such as the dura and the nerve root, but the resultingtransforaminal window available to remove the affected disc, prepare thevertebral endplates, and insert the implant is limited laterally.

[0005] A number of different implants typically used for the traditionalPLIF procedure have been used for the T-PLIF procedure, with varyingsuccess. These include threaded titanium cages, allograft wedges, rings,etc. However, as these devices were not designed specifically for theT-PLIF procedure, they are not shaped to be easily insertable into theaffected disc space through the narrow transforaminal window, and mayrequire additional retraction of nerve roots. Such retraction can causetemporary or permanent nerve damage. In addition, some of theseimplants, such as the threaded titanium cage, suffer from thedisadvantage of requiring drilling and tapping of the vertebralendplates for insertion. Further, the incidence of subsidence in longterm use is not known for such cages. Finally, restoration of lordosis,i.e., the natural curvature of the lumbar spine is very difficult when acylindrical titanium cage is used.

[0006] As the discussion above illustrates, there is a need for animproved implant and instrumentation for fusing vertebrae via thetransforaminal lumbar interbody fusion procedure.

SUMMARY OF THE INVENTION

[0007] The present invention relates to an intervertebral implant(“T-PLIF implant”) and its use during a transforaminal lumbar interbodyfusion procedure. In a preferred embodiment, the T-PLIF implant has anarcuate body with curved, substantially parallel posterior and anteriorfaces separated by two narrow implant ends, and superior and inferiorfaces having a plurality of undulating surfaces for contacting upper andlower vertebral endplates. The undulating surfaces may be projections,such as teeth, of a saw-tooth or pyramidal configuration, or ridgeswhich penetrate the vertebral endplates and prevent slippage. The narrowimplant ends may be rounded or substantially flat. The arcuate implantconfiguration facilitates insertion of the implant via a transforaminalwindow. The implant, which may be formed of allogenic bone, metal, orplastic, may also have at least one depression, such as a channel orgroove, at a first end for engagement by an insertion tool, such as animplant holder. In a preferred aspect, the superior and inferior facesare convex, and the thickness of the implant tapers with its greatestthickness in the middle region between the narrow ends of the implant,i.e., at a section parallel to a sagittal plane, and decreasing towardeach of the narrow ends.

[0008] In another preferred embodiment, the implant is formed of aplurality of interconnecting bodies assembled to form a single unit. Inthis configuration, the plurality of interconnecting bodies forming theT-PLIF implant may be press-fit together and may include at least onepin or screw extending through an opening in the plurality of bodies tohold the bodies together as a single unit. Adjacent surfaces of theplurality of bodies may also have mating interlocking surfaces that aidin holding the bodies together as a single unit.

[0009] In still another preferred embodiment, the present inventionrelates to a kit for implanting an intervertebral implant into anaffected disc space of a patient via a transforaminal window. The kitincludes an implant having an arcuate body with curved, substantiallyparallel posterior and anterior faces separated by two narrow implantends, superior and inferior faces preferably having a plurality ofundulating surfaces, such as projections or teeth, for contacting upperand lower vertebral endplates. The superior and inferior faces maydefine a thickness. Preferably the implant has at least one depressionat a first end for engagement by an insertion tool. The kit may furtherinclude at least one trial-fit spacer for determining the appropriatesize of the implant needed to fill the affected disc space, an insertiontool having an angled or curved neck for holding and properlypositioning the implant during insertion through the transforaminalwindow, and an impactor having an angled or curved neck for properlypositioning the implant within the affected disc space. The face of theimpactor may be concavely shaped to mate with the narrow end of theT-PLIF implant during impaction. The kit may further include a laminaspreader for distracting vertebrae adjacent to the affected disc space,an osteotome for removing facets of the vertebrae adjacent to theaffected disc space to create a transforaminal window, one or morecurettes, angled and/or straight, for removing all disc material fromthe affected disc space, a bone rasp for preparing endplates of thevertebrae adjacent the affected disc space, and a graft implant tool forimplanting bone graft material into the affected disc space. The kit maystill further include a curved guide tool to guide the implant into theaffected disc space.

[0010] In yet another aspect, a method for implanting an intervertebralimplant into an affected disc space of a patient via a transforaminalwindow is described. The transforaminal window is created and bone graftmaterial is inserted into the affected disc space. Using an insertiontool, an implant is inserted into the affected disc space via thetransforaminal window, the implant having an arcuate body with curved,substantially parallel posterior and anterior faces separated by twonarrow implant ends, superior and inferior faces having a plurality ofundulating surfaces for contacting upper and lower vertebral endplates,and preferably at least one depression at a first end for engagement bythe insertion tool. In the present method, the arcuate implantconfiguration facilitates insertion of the implant via thetransforaminal window. The method may further comprise impacting theimplant with an impactor tool to properly position the implant withinthe affected disc space. Either or both the insertion tool and theimpactor tool may be angled to facilitate insertion, alignment,placement and/or proper seating of the implant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a top view of a typical human vertebrae showing thetransforaminal window through which an implant according to the presentinvention is inserted;

[0012]FIG. 2A is a cross-section view of an embodiment of an implantaccording to the present invention;

[0013]FIG. 2B is a side view along the longer axis of the implant ofFIG. 2A;

[0014]FIG. 2C is a cross-section view taken along line 2C-2C of FIG. 2B;

[0015]FIG. 2D is a perspective view of the implant of FIG. 2A;

[0016]FIG. 3A is a partial cross-section view of another embodiment ofan implant according to the present invention;

[0017]FIG. 3B is a partial cross-section view along the longer axis ofthe implant of FIG. 3A;

[0018]FIG. 3C is a cross-section view taken along line 3C-3C of FIG. 3B;

[0019]FIG. 3D is a perspective view of the implant of FIG. 3A;

[0020]FIG. 4 is a perspective view of still another embodiment of theimplant of the present invention;

[0021]FIG. 5 is an axial view of a typical human vertebrae showing theimplant of FIG. 4 in an asymmetric final position.

[0022]FIG. 6 is a posterior view of a section of human spine prior topreparation of the transforaminal window;

[0023]FIG. 7 is a posterior view of a section of human spine with thetransforaminal window prepared;

[0024]FIG. 8A depicts an angled bone curette for use during the T-PLIFprocedure;

[0025]FIG. 8B depicts another angled bone curette for use during theT-PLIF procedure;

[0026]FIG. 8C depicts an angled bone curette removing disc material froman affected disc space;

[0027]FIG. 9A depicts an angled bone rasp for use during a T-PLIFprocedure;

[0028]FIG. 9B depicts an angled bone rasp removing material from anaffected disc space;

[0029]FIG. 10A depicts a trial-fit spacer for use during a T-PLIFprocedure;

[0030]FIG. 10B depicts a trial-fit spacer being inserted into anaffected disc space via a transforaminal window;

[0031]FIG. 11A depicts an implant holder for use during a T-PLIFprocedure;

[0032]FIG. 11B depicts the tips of the implant holder shown in FIG. 11A;

[0033]FIG. 11C depicts a top view of a human vertebrae showing a T-PLIFimplant being inserted with in an implant holder;

[0034]FIG. 11D depicts an posterior view of the human spine showing aT-PLIF implant being inserted with an implant holder;

[0035]FIG. 12 depicts an implant guide tool for use with the T-PLIFimplant;

[0036]FIG. 13A depicts an angled impactor tool for use with the T-PLIFimplant;

[0037]FIG. 13B is a close-up view of the tip of the impactor tool shownin FIG. 13A;

[0038]FIG. 14 is a top view of a typical human vertebrae showing animplant according to the present invention being properly positionedinto an affected disc space using the impactor tool shown in FIG. 13A;and

[0039]FIG. 15 is a top view of the vertebrae of FIG. 1 showing theT-PLIF implant in a final position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] An implant according to the present invention, referred to hereinas a transforaminal posterior lumbar interbody fusion implant (“T-PLIFimplant”), is designed for use as an intervertebral spacer in spinalfusion surgery, where an affected disk is removed from between twoadjacent vertebrae and replaced with an implant that provides segmentalstability and allows for bone to grow between the two vertebrae tobridge the gap created by disk removal. Specifically, the T-PLIF implantis designed for the transforaminal lumbar interbody fusion (T-PLIF)technique, which, as shown in FIG. 1, involves a posterior approach 12,offset from the midline 14 of the spine, to the affected intervertebraldisk space 16. The window 18 available for implant insertion using theT-PLIF technique is limited laterally by the dura 20 and the superiorexiting nerve root (not shown).

[0041] As shown in FIGS. 2A through 2D, in a preferred embodiment, theT-PLIF implant has an arcuate, “rocker-like” body 22 with curvedanterior and posterior faces 24, 26 to facilitate the offset insertionof the implant through the narrow approach window 18 into the diskspace. Preferably, the anterior and posterior faces 24 and 26 aresubstantially parallel, separated by a pair of narrow ends 25. Narrowends 25 may be rounded or blunt. The superior and inferior surfaces 28,30 have projections, such as teeth 32, for engaging the adjacentvertebrae. Teeth 32 on superior and inferior surfaces 28, 30 preferablyprovide a mechanical interlock between implant 22 and the end plates bypenetrating the end plates. The initial mechanical stability afforded byteeth 32 minimizes the risk of post-operative expulsion/slippage ofimplant 10. Teeth 32 may have a saw-tooth shape, where one side of thetooth is perpendicular to the superior or inferior surface, or a pyramidshape, where each tooth has four sides and forms an acute angle with thesuperior or inferior face. Preferably, implant body 22 has at least onechannel or slot 34 on one end of implant 22 for engagement by a surgicalinstrument, such as an implant holder 66 (shown in FIG. 11A). It shouldbe noted that implant 22 may also be configured with a channel 34 ononly one side or without channels altogether. Other known methods forengaging the surgical instruments with the implant, such as a threadedbore for receiving the threaded end of a surgical tool, may also beused.

[0042] As shown in FIG. 2B, thickness 31 of implant 22 is greatest atthe mid-section between the two narrow implant ends 25 and tapersgradually along the longitudinal axis 36 of implant 22 so that it isthinnest at the narrow ends 25 of implant 22. This convex configurationprovides a proper anatomical fit and facilitates insertion of implant 22into the affected disc space. It should be noted that in a preferredembodiment, thickness 31 does not taper along the shorter axis 37 ofimplant 22. Thus, as shown in FIG. 2C for any given cross section takenperpendicular to the longitudinal axis 36 of the implant, the distancebetween the superior and inferior surfaces 28 and 30 remainssubstantially constant. In alternate embodiments, however, thickness 31may taper along shorter axis 37 of implant 22. The dimensions of implant22 can be varied to accommodate a patient's anatomy, and the thicknessof the implant is chosen based on the size of the disk space to befilled. Preferably, implant 22 has a maximum thickness 31 at itsmid-section of about 7.0 to about 17.0 mm, and may be formed of metal,allograft, a metal-allograft composite, a carbon-fiber polymer, purepolymer or plastic. The thickness at the narrow ends 25 of implant 22may range from about 1.5 to about 2.0 mm less than the maximum thicknessat the mid-section. The implant may range from about 26 to about 32 mmin length, and have a width from about 9 to 11 mm. Implant 22, which asshown most clearly in FIG. 2A is symmetric about at least one axis ofrotation 37, is intended for symmetric placement about the midline 14 ofthe spine (see FIG. 19). The arcuate configuration of implant 22facilitates insertion of the implant from the transforaminal approachinto a symmetric position about the midline of the spine so that asingle implant provides balanced support to the spinal column.

[0043] As shown in FIGS. 3A-3D, in an alternate embodiment implant 22may be formed of two or more pieces 38 having interlocking grooves 39and pallets 40 that are press-fit and fastened together with pins orscrews 42. The number and orientation of pins or screws 42 can bevaried. This multi-component configuration may be particularly usefulfor implants formed of allograft bone, since it may be difficult and/orimpractical to obtain a single, sufficiently large piece of allograftfor some applications. In the case of implants formed completely ofartificial (i.e., non-allograft) materials, such as steel, plastic ormetallic or non-metallic polymer, a one-piece implant may be morepractical.

[0044] As in the previous embodiment, the anterior and posterior faces24, 26 are substantially parallel, and, as shown, may be defined byradii of curvature R1 and R2, where R1, for example, may be in the rangeof about 28 mm and R2, for example, may be in the range of about 19 mm.The superior and inferior surfaces 28, 30 are arcuate shaped and theimplant has a thickness 31, which is preferably greatest at a centerportion between narrow ends 25 and gradually tapers becoming thinnest atnarrow ends 25. Tapering thickness 31 may be defined by a radius ofcurvature R3, where R3 for example, may be in the range of about 100 mm.As shown, the component pieces 46, 48 of implant 22 have holes 44 toaccommodate pins or screws 42. Holes 44 are preferably drilled aftercomponent pieces 38 have been stacked one on top of the other. Themultiple pieces 38 are then assembled with screws or pins 42 so thatpractitioners receive the implant 22 as a single, pre-fabricated unit.The upper component piece 46 has an arcuate superior surface preferablywith teeth 32, while its bottom surface is configured with grooves andpallets to interlock with the upper surface of lower component piece 48.The arcuate inferior surface 30 of lower component piece 48 alsopreferably has teeth 32 for engaging the lower vertebral endplate of theaffected disc space. Either or both superior and inferior surfaces 28,30 may have ridges or some other similar form of engaging projection inplace of teeth 32.

[0045] Reference is now made to FIG. 4 which is a perspective view ofanother embodiment an implant. As in the previous embodiment, implant 23has a curved body with substantially parallel arcuate anterior andposterior faces 24, 26, convex superior and inferior surfaces 28, 30contributing to a tapering thickness 31, and channels 34 for engaging asurgical instrument, such as an insertion tool. In this embodiment,implant 23 has a substantially straight or blunted narrow end 50 and acurved narrow end 52 separating parallel, arcuate anterior and posteriorfaces 24, 26. As shown in FIG. 5, the final position of implant 23 indisc space 16 may be asymmetric with respect to midline 14 of thepatient's spine.

[0046] As shown in FIGS. 2A & 3A, and FIG. 11C, the rocker-like shape ofimplant 22 enables the surgeon to insert the implant through the narrowtransforaminal window, typically on the range of about 9.0 mm wide, andseat the implant in the disc space behind the dura without disturbingthe anterior curtain of the disc space. The typical surgical techniquefor the T-PLIF procedure begins with the patient being placed in a proneposition on a lumbar frame. Prior to incision, radiographic equipmentcan assist in locating the precise intraoperative position of the T-PLIFimplant. Following incision, the facets, lamina and other anatomicallandmarks are identified. The affected vertebrae are distracted using alamina spreader or a lateral distractor, both of which are commonlyknown in the art. In the latter case, screws may be inserted into thevertebrae to interface with the lateral distractor. As shown in FIGS. 6& 7, following distraction, the transforaminal window 54 is created byremoving the inferior facet 56 of the cranial vertebrae and the superiorfacet 58 of the caudal vertebrae using one or more osteotomes 59 ofdifferent sizes. A discectomy is performed during which all discmaterial from the affected disc space may be removed using a combinationof straight and angled curettes. Angled curettes, which may beconfigured with rounded profile 60 (FIG. 8A) or a rectangular profile 61(FIG. 8B), enable removal of material on the far side 63 of the discspace opposite transforaminal window 54, as shown in FIG. 8C.

[0047] After the discectomy is complete, the superficial layers of theentire cartilaginous endplates are removed with a combination ofstraight and angled bone rasps. As shown in FIGS. 9A and 9B, angledrasps 62 may be angled to reach far side 63 of the disc space oppositetransforaminal window 54. Rasps 62 expose bleeding bone, but care shouldbe taken to avoid excess removal of subchondral bone, as this may weakenthe anterior column. Entire removal of the endplate may result insubsidence and loss of segmental stability. Next, an appropriately sizedtrial-fit T-PLIF spacer/template 64, shown in FIGS. 10A and 10B, may beinserted into the intervertebral disc space using gentle impaction, todetermine the appropriate implant thickness for the disc space to befilled. Fluoroscopy can assist in confirming the fit of the trialspacer. If the trial spacer 64 appears too loose/too tight, the nextlarger/smaller size trial spacer should be used until the most securefit is achieved. For example, if a trial fit spacer with a maximumthickness of 11 mm is too loose when inserted into the disc space, aphysician should try the 13 mm thick spacer, and so on. Trial fitspacers preferably range in height from about 7 mm to about 17 mm.

[0048] Upon identifying and removing the best fitting trial spacer, aT-PLIF implant of appropriate size is selected. At this time, prior toplacement of the T-PLIF implant, bone graft material, such as autogenouscancellous bone or a bone substitute, should be placed in the anteriorand lateral aspect of the affected disc space. As shown in FIGS. 11C and11D, T-PLIF implant 22 is then held securely using a surgical instrumentsuch as implant holder 66 (shown more clearly in FIG. 11A), whichengages the channels or slots 34 at one end of implant 22. The tips 67of implant holder 66 may be curved or angled to mate with curved implant22 and facilitate insertion of implant 22 into disc space 16. T-PLIFimplant 22 is then introduced into the intravertebral disc space 16 viathe transforaminal window, as shown in FIG. 11D. A guide tool having acurved blade 68 (shown in FIG. 12) to match the curvature of theanterior face of implant 22 may be used to properly guide the implantinto affected disc space 16. Slight impaction may be necessary usingimplant holder 66 (shown in FIG. 11A) or an impactor tool 70 (shown inFIG. 13A) to fully seat the implant. As shown in FIGS. 13A & 13B,impactor tool 70 may also be curved or angled to facilitate seating ofthe implant through the narrow transforaminal window. Also, the face 71of impactor 70 may be concavely shaped to mate with the end of implant22, as shown in FIG. 14. Once the T-PLIF implant is in the desired finalposition, such as the symmetric final position shown in FIG. 15 or theasymmetric position shown in FIG. 5, implant holder 66, and possiblyguide tool 68, is removed and additional bone graft material 73 may beinserted. Preferably, T-PLIF implant 22 should be recessed from theanterior edge 72 of the vertebral body. As shown in FIG. 15, thecurvature of anterior face 24 of implant 22 is substantially the same asthe curvature of anterior edge 72 of disc space 16. In the symmetricseated position shown in FIG. 15, a single T-PLIF implant 22 providesbalanced support to the spinal column about the midline of the spine.

[0049] While certain preferred embodiments of the implant have beendescribed and explained, it will be appreciated that numerousmodifications and other embodiments may be devised by those skilled inthe art. Therefore, it will be understood that the appended claims areintended to cover all such modifications and embodiments which comewithin the spirit and scope of the present invention.

What is claimed is:
 1. An intervertebral implant comprising: a bodyhaving a substantially curved anterior face, a substantially curvedposterior face, a pair of narrow ends separating the posterior andanterior faces; a superior surface and an inferior surface forcontacting upper and lower vertebral endplates, the superior andinferior surfaces defining a thickness of the implant; and first andsecond non-threaded, horizontal channels configured and adapted forengagement by an implant insertion tool, the first channel disposedalong at least a portion of the curved posterior face and the secondchannel disposed along at least a portion of the curved anterior face;wherein the first and second channels are curved with the anterior andposterior faces.
 2. The implant of claim 1, wherein the anterior face isconvex and the posterior face is concave.
 3. The implant of claim 2,wherein the anterior face and the posterior face are substantiallyparallel.
 4. The implant of claim 3, wherein the anterior face has aradii of curvature R1, the posterior face has a radii of curvature R2,wherein R1 is approximately 28 millimeters and R2 is approximately 19millimeters.
 5. The implant of claim 1, wherein the thickness of theimplant is greatest approximately at a mid-section of the implantbetween the ends of the implant.
 6. The implant of claim 5, wherein thethickness of the implant tapers from the mid-section of the implant tothe ends of the implant.
 7. The implant of claim 5, wherein the superiorand inferior surfaces have a convex-arcuate shape with a radii ofcurvature of approximately 100 mm.
 8. The implant of claim 7, whereinthe thickness of the implant does not taper along a transverse axis ofthe implant so that the distance between the superior and inferiorsurfaces remain substantially constant for any given cross section takenperpendicular to the longitudinal axis of the implant.
 9. The implant ofclaim 5, wherein the thickness of the implant at the mid-section isapproximately 7.0 millimeters to approximately 17.0 millimeters and thethickness at the ends of the implant is approximately 1.5 millimeters toapproximately 2.0 millimeters less than the thickness at themid-section.
 10. The implant of claim 1, wherein the implant has alongitudinal length measured along a longitudinal axis of the implantfrom approximately 26 millimeters to approximately 32 millimeters and atransverse width from approximately 9 millimeters to approximately 11millimeters.
 11. The implant of claim 1, wherein the implant is formedof a plurality of interconnecting bodies assembled to form a singleunit.
 12. The implant of claim 11, wherein the plurality ofinterconnecting bodies have interlocking grooves and pallets topress-fit the bodies together.
 13. The implant of claim 12, furthercomprising at least one pin extending through an opening in theplurality of bodies to hold the bodies together as a single unit. 14.The implant of claim 12, further comprising at least one screw extendingthrough an opening in the plurality of bodies to hold the bodiestogether as a single unit.
 15. The implant of claim 11, wherein theimplant is formed from exactly two pieces, the first piece has aninferior surface for engaging an adjacent vertebral and a top surface,the second piece has a superior surface for engaging an adjacentvertebral and a bottom surface for interlocking with the top surface ofthe first piece so that the pieces are stacked one on top of the other.16. The implant of claim 15, wherein the first and second pieces areformed allograft bone.
 17. The implant of claim 1, wherein the implantis formed from the group consisting of metal, allograft, a metalallograft composite, a carbon-fiber polymer, a polymer, or plastic. 18.The implant of claim 1, further comprising a plurality of projections onthe superior and inferior faces for engaging the adjacent vertebrae. 19.The implant of claim 18, wherein the projections are teeth for providinga mechanical interlock between the superior and inferior surfaces of theimplant and the end plates of the adjacent vertebral.
 20. The implant ofclaim 1, wherein at least one of the narrow ends is rounded.
 21. Theimplant of claim 1, wherein at least one of the narrow ends issubstantially straight.
 22. An intervertebral implant comprising: a bodyhaving a curved posterior face and a curved anterior face, both curvedposterior and anterior faces extending in the direction of alongitudinal axis of the implant body; a pair of narrow end facesseparating the posterior and anterior faces; superior and inferior facesfor contacting upper and lower vertebral endplates, the superior andinferior surfaces defining a thickness of the implant; and first andsecond non-threaded, horizontal channels configured and adapted forengagement by an implant insertion tool, the first channel disposedalong at least a portion of the curved posterior face and the secondchannel disposed along at least a portion of the curved anterior face;wherein the posterior face and the anterior face are both curvedsubstantially from one of the end faces to the other end face.
 23. Theimplant of claim 22, wherein the first and second channels are curvedwith the anterior and posterior faces.
 24. The implant of claim 23,wherein the anterior face is convex and the posterior face is concave.25. The implant of claim 24, wherein the anterior face and the posteriorface are substantially parallel.
 26. The implant of claim 24, whereinthe anterior face has a radii of curvature R1, the posterior face has aradii of curvature R2, wherein R1 is approximately 28 millimeters and R2is approximately 19 millimeters.
 27. The implant of claim 22, whereinthe thickness of the implant is greatest approximately at a mid-sectionof the implant between the end faces of the implant.
 28. The implant ofclaim 27, wherein the thickness of the implant tapers from themid-section of the implant to the end faces of the implant.
 29. Theimplant of claim 27, wherein the superior and inferior surfaces have aconvex-arcuate shape with a radii of curvature of approximately 100 mm.30. The implant of claim 29, wherein the thickness of the implant doesnot taper along a transverse axis of the implant so that the distancebetween the superior and inferior surfaces remain substantially constantfor any given cross section taken perpendicular to the longitudinal axisof the implant.
 31. The implant of claim 27, wherein the thickness ofthe implant at the mid-section is approximately 7.0 millimeters toapproximately 17.0 millimeters and the thickness at the end faces of theimplant is approximately 1.5 millimeters to approximately 2.0millimeters less than the thickness at the mid-section.
 32. The implantof claim 22, wherein the implant has a longitudinal length measuredalong a longitudinal axis of the implant from approximately 26millimeters to approximately 32 millimeters and a transverse width fromapproximately 9 millimeters to approximately 11 millimeters.
 33. Theimplant of claim 22, wherein the implant is formed of a plurality ofinterconnecting bodies assembled to form a single unit.
 34. The implantof claim 33, wherein the plurality of interconnecting bodies haveinterlocking grooves and pallets to press-fit the bodies together. 35.The implant of claim 34, further comprising at least one pin extendingthrough an opening in the plurality of bodies to hold the bodiestogether as a single unit.
 36. The implant of claim 34, furthercomprising at least one screw extending through an opening in theplurality of bodies to hold the bodies together as a single unit. 37.The implant of claim 33, wherein the implant is formed from exactly twopieces, the first piece has an inferior surface for engaging an adjacentvertebral and a top surface, the second piece has a superior surface forengaging an adjacent vertebral and a bottom surface for interlockingwith the top surface of the first piece so that the pieces are stackedone on top of the other.
 38. The implant of claim 37, wherein the firstand second pieces are formed allograft bone.
 39. The implant of claim22, wherein the implant is formed from the group consisting of metal,allograft, a metal allograft composite, a carbon-fiber polymer, apolymer, or plastic.
 40. The implant of claim 22, further comprising aplurality of projections on the superior and inferior faces for engagingthe adjacent vertebrae.
 41. The implant of claim 40, wherein theprojections are teeth for providing a mechanical interlock between thesuperior and inferior surfaces of the implant and the end plates of theadjacent vertebral.
 42. The implant of claim 22, wherein at least one ofthe end faces is convex.
 43. The implant of claim 22, wherein at leastone of the end faces is substantially straight.
 44. An intervertebralimplant comprising: a body having a substantially convex anterior face,a substantially concave posterior face, wherein the convex anterior facehas a radii of curvature of approximately 28 millimeters, the concaveposterior face has a radii of curvature of approximately 19 millimeters;a pair of narrow ends separating the posterior and anterior faces; asuperior convex surface and an inferior convex surface for contactingupper and lower vertebral endplates, the superior and inferior surfacesdefining a thickness of the implant, wherein the thickness of theimplant is greatest approximately at a mid-section of the implantbetween the ends of the implant, and the superior and inferior surfaceshave a radii of curvature of approximately 100 mm; so that the thicknessof the implant at the mid-section is approximately 7.0 millimeters toapproximately 17.0 millimeters and the thickness at the ends of theimplant is approximately 1.5 millimeters to approximately 2.0millimeters less than the thickness at the mid-section; and first andsecond non-threaded, horizontal channels configured and adapted forengagement by an implant insertion tool, the first channel disposedalong at least a portion of the curved posterior face and the secondchannel disposed along at least a portion of the curved anterior face;wherein the first and second channels are curved with the anterior andposterior faces.
 45. An intravertebral implant for maintaining a desiredspace between vertebral bodies comprising: a solid body having a convexanterior face, the anterior face shaped to match the shape of theanterior perimeter of the vertebrae; a concave posterior face; arcuateconvex end faces connected between the posterior and anterior faces, theposterior and anterior faces being at least double the length of the endfaces and the posterior and anterior faces each having at least onecurved channel extending from one of the end faces toward the other endface configured and adapted to be engageable with an insertion tool; andsuperior and inferior faces for contacting upper and lower vertebral endplates, the superior and inferior faces having teeth, wherein theimplant is configured and adapted to be inserted into the disc spacethrough a transforaminal window formed in the posterior of the spinalcolumn and positioned behind the dura between vertebral end plates tomaintain balanced support about the midline of the spine.